Utility interactive point of connection: Section 64-112 • David Pilon
December 12, 2016 - An interactive power production system operates in conjunction with—and can deliver power to—another power system, such as the utility. While this could involve a standard generator, I will focus on renewable energy systems.
A number of concerns arise with the interconnection of renewables. CE Code Section 64 for renewables talks about the utility point of interconnection, and the first Subrule directs us to Section 84.
Section 84 tells us any interconnected system shall be provided with protection against backfeed, equipped to maintain a synchronous condition with the utility, and disconnect all ungrounded conductors in the event the supply authority loses voltage in one or more phases. This requirement must be designed into the system and, as such, most inverters are constructed with power monitoring as part of their function (including backfeed prevention, synchronization, etc.).
The other part of the equation is to ensure the system’s safe operation and maintenance. This requires the conductors and equipment—which are energized from both directions—to have overcurrent protective devices (OCPDs) for each source of supply. In a panel, for instance, the solar feed must provide an OCPD just as the main supply entering the panel has overcurrent protection. When a transformer is added to the mix, the OCPDs must be calculated for each side of the transformer (in accordance with Section 26) by considering first one side, then the other, as the primary.
The system also requires a disconnecting means from the power production source as well as the supply authority. These requirements are a bit generic, as they cover an array of systems and require a level of flexibility. In general, these disconnects must be capable of being energized from both directions, indicate whether they are open or closed, and have verifiable contact operation to the satisfaction of the supply authority. In some instances, these disconnecting means may be fulfilled by the requirements for the OCPDs.
Having the means to isolate equipment fed from both directions is also required. This may be the disconnecting means described above but, due to location or some other nuance, you may require additional disconnects to meet the Rule’s intent. Check with your local AHJ before assuming your solution fits the bill for two or more functions.
Now that we’re aware of the constraints involved with an interactive system, we can head back to 64-112. The most common installation we are currently seeing is a “utility interactive” connection, where the renewable system feeds into an inverter then connects through the consumers’ distribution equipment to the utility in accordance with 64-112(4).
Each panelboard is to be marked to indicate more than one available source, and display a warning that all disconnects must be open to safely de-energize the equipment. The renewable energy breaker shall be positioned at the opposite end of the busbar from the utility supply breaker; a label must be affixed to clearly identify the breaker along with a warning that it is not to be relocated.
Finally, we need to ensure the panelboard bussing does not carry excessive amounts of current. Some or all of the generated power may be used by the loads supplied by the panelboard.
When the panelboard is on a dwelling unit, then we allow the bussing to be supplied by breakers totalling no more that 125% of the buss’ rating. An average 200A single-phase panel has a buss rating of 225A; therefore, the total ampacity of the two breakers supplying the buss shall not exceed 281A.
Dwelling units aside, the same panel can only be supplied by a total breaker ampacity not exceeding 120% of the buss or 270A. This is important, as the dwelling unit is considered a non-continuous load. Remember to check with your AHJ and utility, as they may have other requirements for interactive systems.
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